Low-emissivity coatings are well known in the present art. Typically, they include one or more infrared-reflective films each positioned between two or more transparent dielectric films. The infrared-reflective films, which are typically conductive metals such as silver, gold, or copper, reduce the transmission of radiant heat through the coating (e.g., by reflecting infrared radiation). The transparent dielectric films are used primarily to reduce visible reflectance and to control other coating properties, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium, as well as nitrides, such as silicon nitride.
In addition to the infrared reflection provided by low-emissivity coatings, these coatings can be engineered to provide desired shading properties. As is well known, the solar heat gain coefficient (SHGC) of a window is the fraction of incident solar radiation that is admitted through a window. There are a number of applications where low solar heat gain windows are of particular benefit. In warm climates, it is especially desirable to have low solar heat gain windows. For example, solar heat gain coefficients of about 0.4 and below are generally recommended for buildings in the southern United States. Further, windows that are exposed to a lot of undesirable sun benefit from having a low solar heat gain coefficient. For example, windows on the east or west side of a building tend to get a lot of sun in the morning and afternoon. For applications like these, the solar heat gain coefficient of a window plays a vital role in maintaining a comfortable environment within the building. Thus, it is beneficial to provide windows of this nature with coatings that establish a low solar heat gain coefficient (i.e., high shading ability coatings).
A tradeoff is sometimes made in high shading ability coatings whereby the films selected to achieve a low SHGC have the effect of increasing the visible reflectance to a higher level than is ideal. As a consequence, windows bearing these coatings may have a somewhat mirror-like appearance. It would be desirable to provide a high shading ability coating that has sufficiently low visible reflectance to obviate this mirror-like appearance problem.
In addition to having undesirably high visible reflectance, the transmitted and reflected colors of conventional high shading ability coatings tend not to be ideal. For example, these coatings commonly exhibit hues that are more red and/or yellow than is desired. To the extent a coating has a colored appearance, it is pleasing if the coating exhibits a transmitted and/or reflected hue that is blue or blue-green. The chroma of these coatings tends also to be greater than is desired. In most cases, it is preferable to provide a coating that is as color neutral (i.e., colorless) as possible. Thus, the reflected and transmitted colors of conventional low solar heat gain coatings tend to be less than ideal, both in terms of hue and chroma.
It is extremely challenging to produce any low-emissivity coating that exhibits the desirable properties described herein. These desirable coating properties are even more difficult to achieve in a temperable coating. Glass tempering typically involves heating glass to elevated temperatures on the order of 680 degrees C. and above (e.g., commonly reaching about 695 degrees C.). As is well known in the art, the properties of low-emissivity coatings tend to change significantly as a result of tempering. However, it is incredibly difficult to predict the exact changes that may occur in such a coating during tempering. As a result, the design of temperable coatings is extremely unpredictable. Thus, it would be particularly desirable to provide a high shading ability low-emissivity coating that exhibits desirable properties even after being tempered.